A one-part adhesive is generally supplied in the liquid form and coated onto an adherend, as by a brush or roller, which is subsequently joined to another adherend. After the joining, the adhesive solidifies as a result of evaporation of a solvent or growth of its molecular weight, so that the adherends are firmly bonded to each other.
General liquid adhesives exhibit high bond strength after completion of curing but need a relatively long time to complete the reaction. Accordingly, adherends must be provisionally held in contact with each other in some way until the liquid adhesive solidifies. This results in the insufficient workability.
Solvent-borne adhesives need a drying oven to vaporize a solvent and adversely affect working environments, which has been a problem.
As a solution to the above-described problems, Japanese Patent Laying-Open No. Sho 61-31418 discloses a one-part urethane adhesive. The one-part urethane adhesive exhibits high bond strength because it cures by a moisture-curing reaction. However, the one-part urethane adhesive fails to provide sufficient initial bond strength since its moisture-curing reaction proceeds insufficiently just after it has combined adherends together, and exhibits insufficient heat resistance just after it has combined adherends together.
Japanese Patent Laying-Open No. Sho 64-24821 discloses a photocurable polyurethane composition which contains a polyurethane oligomer having an active isocyanate group, a UV-curable organic compound having a hydroxyl group, and a photoinitiator.
Japanese Patent Publication No. Hei 7-103356 discloses a bonding method utilizing a photocurable polyurethane composition which contains an isocyanate compound, a free-radically photopolymerizable compound and a free-radically photopolymerizable monomer.
In cases where the preceding photocurable polyurethane compositions utilizing a free-radical photopolymerization reaction are used as adhesives, photocurable components must be increased in contents and crosslinked by irradiation to improve heat resistance of the adhesives immediately after combination of adherends. However, when the photocurable components are increased in contents and crosslinked by irradiation, adhesion of the composition to the adherends is lowered to result in the problematic reduction in initial bond strength and bond strength after cure.